AD conversion apparatus, solid-state imaging apparatus, and imaging system

Abstract

Provided is an AD conversion apparatus including: a reference signal generating circuit configured to output a first reference signal and a second reference signal, whose voltages change with time; a comparison circuit configured to perform a comparison between a voltage of the analog signal and the voltage of the first reference signal; a control circuit configured to generate and output digital data based on the comparison; a digital-to-analog converter configured to generate, using the second reference signal, a signal whose voltage changes with time from a comparison base voltage, the comparison base voltage being based on the digital data, and configured to output the signal to the comparison circuit; and a counter configured to generate a count value by measuring an elapsed time. The comparison circuit is further configured to perform a comparison between the voltage of the analog signal and the signal output from the digital-to-analog converter.

What is claimed is: 
     
       1. An analog-to-digital (AD) conversion apparatus configured to convert an analog signal into a digital signal, comprising:
 a reference signal generating circuit configured to output a first reference signal and a second reference signal whose voltages change with time; 
 a comparison circuit configured to perform a comparison between a voltage of the analog signal and the voltage of the first reference signal; 
 a control circuit configured to generate and output digital data based on the comparison; 
 a digital-to-analog converter configured to generate, using the second reference signal, a signal whose voltage changes with time from a comparison base voltage, the comparison base voltage being based on the digital data, and configured to output the signal to the comparison circuit; and 
 a counter configured to generate a count value by measuring an elapsed time, 
 wherein the comparison circuit is further configured to perform a comparison between the voltage of the analog signal and the signal output from the digital-to-analog converter, 
 wherein the counter obtains a first count value by measuring a time from when changing of the voltage of the first reference signal with time starts, to when a magnitude relationship between the voltage of the analog signal and the voltage of the first reference signal, which are input to the comparison circuit, is changed, 
 wherein the digital data has a value based on the first count value. 
 
     
     
       2. The AD conversion apparatus according to  claim 1 , wherein the counter obtains a second count value by measuring a time from when changing of the voltage of the second reference signal with time starts, to when a magnitude relationship between the voltage of the analog signal and the voltage of the second reference signal, which are input to the comparison circuit, is changed. 
     
     
       3. The AD conversion apparatus according to  claim 2 , wherein the analog signal is converted into the digital signal by combining the digital data based on the first count value as a higher-order bit and the digital data based on the second count value as a lower-order bit. 
     
     
       4. The AD conversion apparatus according to  claim 3 , wherein the control circuit sets a value obtained by shifting the first count value by at least 1 bit as a value of the high-order bit. 
     
     
       5. The AD conversion apparatus according to  claim 1 ,
 wherein the comparison circuit has one input terminal configured to receive the analog signal, and another input terminal configured to selectively receive either one of an output signal of the digital-to-analog converter and an output signal of the reference signal generating circuit, and 
 wherein, when the reference signal generating circuit is selected so that the first reference signal is input to the comparison circuit from the reference signal generating circuit, the comparison circuit compares the analog signal and the first reference signal. 
 
     
     
       6. The AD conversion apparatus according to  claim 1 ,
 wherein the comparison circuit has one input terminal configured to receive the analog signal, and another input terminal configured to receive an output signal of the digital-to-analog converter, and 
 wherein, when the first reference signal output from the reference signal generating circuit is input to the comparison circuit via the digital-to-analog converter, the comparison circuit compares the analog signal and the first reference signal. 
 
     
     
       7. The AD conversion apparatus according to  claim 6 , wherein, before the voltage of the analog signal and the voltage of the first reference signal are compared to each other, the digital-to-analog converter inputs a voltage generated with a value of a most significant bit being 1 to the comparison circuit so that the voltage is compared to the analog signal, to thereby determine a value of a most significant bit of the analog signal. 
     
     
       8. The AD conversion apparatus according to  claim 1 ,
 wherein the comparison circuit has one input terminal configured to receive the analog signal and an output signal of the digital-to-analog converter, 
 wherein the AD conversion apparatus further comprises:
 a switch connected between another input terminal of the comparison circuit and an output terminal of the comparison circuit; and 
 a capacitor having one terminal connected to the another input terminal, and 
 
 wherein, when the first reference signal is input to another terminal of the capacitor, the comparison circuit compares the analog signal and the first reference signal. 
 
     
     
       9. The AD conversion apparatus according to  claim 8 ,
 wherein, under a state in which the switch is turned on, the analog signal is input to the comparison circuit, and an offset voltage of the comparison circuit is held in the capacitor, and 
 wherein, after the switch is turned off, the first reference signal is input to the comparison circuit. 
 
     
     
       10. The AD conversion apparatus according to  claim 1 , wherein, before the comparison circuit compares the voltage of the analog signal and the voltage of the first reference signal, the comparison circuit compares a base signal of a signal source configured to supply the analog signal and the second reference signal to convert the base signal into the digital signal. 
     
     
       11. The AD conversion apparatus according to  claim 10 , wherein, in the comparison between the base signal and the second reference signal, the second reference signal has an offset voltage added thereto. 
     
     
       12. The AD conversion apparatus according to  claim 1 , wherein the first reference signal is larger than the second reference signal in voltage change rate with respect to time. 
     
     
       13. The AD conversion apparatus according to  claim 1 , wherein the second reference signal is generated by dividing the voltage of the first reference signal. 
     
     
       14. The AD conversion apparatus according to  claim 1 , wherein the digital-to-analog converter comprises a capacitance type digital-to-analog converter comprising a plurality of capacitors having binary weighted capacitance values. 
     
     
       15. The AD conversion apparatus according to  claim 1 , wherein the digital-to-analog converter comprises an R-2R ladder type digital-to-analog converter comprising a resistor having a first resistance value and a resistor having a resistance value that is twice as large as the first resistance value, which are connected in a ladder configuration. 
     
     
       16. A solid-state imaging apparatus, comprising:
 a pixel part configured to output an analog signal corresponding to incident light; and 
 the AD conversion apparatus according to  claim 1  configured to convert the analog signal into a digital signal. 
 
     
     
       17. An imaging system, comprising:
 a pixel part configured to output an analog signal corresponding to incident light; 
 the AD conversion apparatus according to  claim 1  configured to convert the analog signal into a digital signal; and 
 a signal processing unit configured to process the digital signal.